A boride, in which, in general, the boron atoms bond firmly to each other, has a high hardness, and its melting point or decomposition temperature is high. Accordingly, heretofore, a boride has been used for a heat-resistant material or an abrasion-resistant material. There are known many borides having electrically-specific characteristics such as semiconductor characteristics, thermal electron emission characteristics, superconductivity characteristics, etc. Lanthanum hexaboride (LaB6) has been put into practical use for a hot-cathode material, and magnesium dibromide (MgB2) is specifically noted as a superconductive material.
The electric characteristics of a boride strongly depend on the crystal structure thereof; and a boride having a small boron content exhibits electroconductivity, while a crystal that contains a structure of a boron icosahedron (B12 icosahedral cluster) has semiconductive electric characteristics.
Heretofore, as borides, there are known binary borides composed of two component elements such as the above-mentioned LaB6 and MgB2, and also B4C, BN, etc.; and polynary borides with three component elements. As the borides comprising a B12 icosahedral cluster, many substances have been discovered, for example, M-Al—B (where M means an alkali metal or an alkaline earth element) (for example, see PTL 1), RE1-xB12Si4-y (RE means one or more rare earth elements selected from Y, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; x and y are each fall within a range of 0≦x≦7 and 0≦y≦2) (for example, see PTL 2), etc.
In addition to application development of such already-existing borides, research and development of a further wide variety of borides to thereby develop highly-functional materials having any other novel function heretofore unknown and capable of being used for various applications has become an important theme.
On the other hand, a boride or a boron-containing carbide is a substance that is difficult to sinter since, in general, the covalent character thereof is strong. At present, as a method for producing a sintered product of a boride such as typically B4C, there is employed a hot isostatic press sintering method (HIP method) or a spark plasma sintering method (SPS method). In any case, the method requires high-temperature and high-pressure conditions, and the polycrystalline products capable of being produced by the sintering method are limited to those having a simple form. In addition, a boride that is a superhard material is difficult to work after production of the polycrystalline products thereof, and the limitation on the form thereof is a significant bar to industrial application of the material.
Apart from the above-mentioned pressure sintering method, a pressureless sintering method using a sintering promoter such as alumina, tungsten carbide or the like has been developed (for example, see NPL 1, 2). However, precipitation of the second phase to be caused by the sintering promoter used in such a sintering method may have some negative influence on the mechanical behavior of borides. In addition, as being carried out at a high temperature around 2000° C., the pressureless sintering method has another problem in that the energy cost thereof is high.